Inkjet printer devices generally incorporate one or more inkjet cartridges, often called “pens”, which shoot drops of ink onto a page or sheet of print media. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat.
Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. The pens are usually mounted on a carriage, which is arranged to scan across a slider rod that traverses a print zone, in which a sheet of print media may be located. As the carriage traverses the print zone, the pens print a series of individual drops of ink on the print media forming a band or “swath” of an image, such as a picture, chart or text. The print media is subsequently moved relative to the carriage, so that a further swath may be printed adjacent to the earlier swath. By a repetition of this process, a complete printed page may be produced in an incremental manner.
In order to generate high quality printed output, it is necessary that the ink drops from the individual pens are accurately applied to the print media. This is made possible by accurately measuring the position of the carriage as it traverses the print media. This is generally achieved using an encoder strip or codestrip, which is arranged parallel to the scan direction of the carriage. Such a codestrip is usually made from a plastics material such as Mylar™, upon which a series of graduations or marks are recorded. The graduations, which may be recorded using a laser plotter, give rise to local variations in the properties (such as optical properties) of the codestrip. An optical sensor mounted on the carriage may be used to sense the optical variations in the codestrip as the carriage moves relative to it. The output of the sensor may be used by a microprocessor associated with the printer device to generate position and speed information relating to the carriage.
Currently, the length of such codestrips is limited by the high cost of the machinery required to make manufacture them. However, as demand requires scanning printers with increasingly wide scan axes, longer codestrips are required. Due to the high resolution of the graduations recorded on a codestrip, it is not generally practicable to join two codestrips end-to-end to increase their usable length without causing substantial positioning errors in the region of the join. This is because it is generally not practicable to ensure that the last graduation of one codestrip is separated from the first graduation of the next codestrip by the correct distance.
This distance, if too great, may cause a gross positioning error in the region of a gap between the two codestrips, where no graduations are present. Furthermore, whether the distance is too great or too small, the signals generated by a sensor when reading one codestrip may be phase shifted relative to the signals generated when reading the next codestrip. Of course, both of these factors will cause carriage position measurement errors.
It would therefore be desirable to provide a hard copy device and method, which addresses the problems of the prior art.